This worry spans all categories. As one would expect, it is highest among those earning $25,000 a year or less -- 72%. But a full 36% of those earning $100,000 or more are worried about making payments.

Various studies have shown that student debt prevents people from buying homes, getting married and saving for retirement. A high level of worry can make these problems even worse, even if the debtor is able to make payments.

Student loans are often called "good" debt", because education is an investment that leads to higher earnings and more job flexibility. But deep in the Urban Institute report is a figure that its not so simple:

"On average, the benefits of post-secondary education outweigh the costs for people able to complete their degrees, although roughly half do not complete their degrees."

The report says that half of the students who failed to get the BA degrees they'd expected to earn had student debt averaging $14,457.

The lesson is clear: Don't pile on debt for a college degree unless you'll get the education and credentials that will make it pay. Piling on debt only to drop out turns good debt into bad, a category that includes things such as credit card debt for nights on the town.

The report lists several warning signs, or "risk factors," that increase the likelihood of dropping out. Students who are not well prepared, for instance, have trouble keeping up and drop out or flunk out.

Those who take a break between high school and college are also at higher risk of not completing their college programs. Some have too little commitment, while others may be too challenged by the finances, even if loans are available.